1. Field of the Invention
The present invention relates to a continuous heat treatment for producing a non-ageing cold rolled steel sheet or strip (hereinafter called simply strip) having excellent deep-drawability, and so far as steel compositions and conditions of hot rolling are within the scope of the present invention as defined hereinbelow, it is possible to produce a non-ageing cold rolled steel strip having excellent deep-drawability by a continuous heat treatment without an over-ageing treatment.
2. Description of Prior Art
According to prior arts for producing a cold rolled steel strip by a continuous annealing, the temperature of such continuous annealing is maintained not so high, and the annealing time is short so that it is possible to produce SPCC grade (JIS G3141) or SPCD grade (JIS G3141) only when the additional subsequent over-ageing treatment is performed, but there is no prior art which can obtain a non-ageing deep-drawing grade (SPCE: JIS G3141). Thus, there has never been developed an art which can produce at a low production cost a non-ageing deep-drawing steel strip by a continuous heat treatment without an over-ageing treatment.
Conventionally, a non-ageing cold rolled steel strip having excellent deep-drawability has been produced by cold rolling a killed steel and then box-annealing the cold rolled strip. However, the box-annealing requires several days from the heating to the cooling and thus is very disadvantageous in respect to the production efficiency.
In order to eliminate the disadvantage of the box-annealing, a continuous annealing method has been developed for production of a deep-drawing cold rolled steel strip, but this method requires very strict limitations in the steel composition such as addition of titanium etc. and lowering of the carbon content. Further this conventional continuous annealing method requires an over-ageing treatment after the annealing in order to obtain desired results, so that the annealing furnace requires a very long line, thus causing considerable increase in the capital cost, and the material quality obtainable by this conventional continuous annealing method has been found to be inferior to that obtainable by the box-annealing method.
Various methods have been known for production of non-ageing deep-drawing cold rolled steel strip.
For example, U.S. Pat. No. 3,522,110 discloses a method to fix with titanium the carbon and nitrogen which are causes of the ageing. According to embodiment of this prior art, the carbon content is lowered to a range from 0.003 to 0.017 for example, and titanium is added in an amount 4 times larger than the carbon content, and the steel is hot rolled at 780.degree. C. or higher, acid-pickled, cold rolled with 30% or larger reduction and box-annealed at a temperature lower than 900.degree. C. or continuously annealed between 750.degree. C. and 1000.degree. C. The r value obtainable by this prior art is very high and the resultant ageing index value is markedly low, thus providing a non-ageing cold rolled steel strip having excellent deep-drawability.
U.S. Pat. No. 3,821,031 discloses production of a deep-drawing cold rolled steel strip from Al-killed steel containing no titanium by a continuous annealing. According to this prior art, the carbon content in the steel is lowered to 0.004%, and the steel is hot rolled, coiled at a relatively high temperature between 670.degree. C. and 700.degree. C., cold rolled, continuously annealed in a soaking temperature ranging from 740.degree. C. to 780.degree. C. for 60 seconds, and held in a slow cooling temperature ranging from 520.degree. C. to 400.degree. C. for 80 seconds to effect over-ageing.
The r value obtainable by this prior art ranges from 1.42 to 1.46 and the ageing index value ranges from 4.2 to 5.0 kg/mm.sup.2, thus inferior to those obtainable by U.S. Pat. No. 3,522,110, in which titanium is added to the steel.
The present inventors have made extensive researches and studies for producing the deep-drawing grade (SPCE) by a continuous annealing, and for simplifying and shortening the continuous annealing.